My sheltie has always had a pink tummy. We recently took her into the vet to get spayed and they of course shaved her tummy. It's been a couple months since and her fur is growing back on her tummy. But the strange thing is her skin has slowly been changing color on her tummy from the normal pink to a purpleish blackish color. It's not painful to her and I normally would have thought perhaps sun exposure has changed the color but her tummy hasn't been in the sun. Is this normal or something to take her to the vet for?

How old is she?

Quite often dogs pigment won't fill in until they are older, sometimes up to 1 year of age.
It's more common in their noses and the rims of their eyes but not unheard of on the stomach.

Does the skin look/feel OK, not dry or scaly?

My Shih Tzu's belly changed color as she aged. Our vet explained it kind of like freckles.

She's almost 7. And yes actually, I've noticed her skin seems to be dry and a bit flaky.
I wondered if it was a natural discoloration similar to freckles, but I want to be on the safe side. Thanks for all the help you're giving me!

Try putting a little moisturizer on her tummy where it feels dry.
Sorbolene is good if you have it where you live.

Dryness can be caused by a lot of things from allergies to just generally being dry. If she hasn't had a problem before now then it could be the fact that the skin on her tummy is coming into contact with things when she lays down more than it did when she had hair there and it's irritating it slightly.